1. Field of the Invention
The invention relates to digital integrated optical modulators. More particularly, it pertains to a modulator for a fiber optic signal transmission or measuring device.
2. Description of the Prior Art
A digital phase modulator with increased resolution for a fiber optic signal transmission or measuring device is known from patent document DE 103 078 525 B4. As shown, the resolution of digital electrodes is increased to a range of 12 to 13 bits by combining binary and non-binary electrodes to form a coarse modulator and a fine modulator with the chip length required being the same as a binary 9-bit arrangement.
Although the above-described solution allows increased resolution of 12 or 13 bits while maintaining the total chip length, the resolution cannot be further increased with the described device because the total chip length cannot be kept in a configuration such as taught in DE 103 07 525 B4. This is due to the fact that total chip length required depends on the minimum overall length of the smallest electrode (LSB electrode). The bit weight for a very short LSB electrode (in particular for a binary/non-binary arrangement) can be precisely determined only with considerable difficulty. For this reason, certain minimum lengths are required for the LSB electrode. The length of the LSB electrode, in turn, also accounts for the length of the longest or most significant bit electrode (MSB electrode) which must be twice the length of the second longest electrode.
A digital phase modulator whose resolution is increased from 9 to 11 bits without alteration of chip length is taught in WO-A-2004/074914. Such reference teaches the arrangement of the electrodes into a plurality of groups of three each along the light guiding section with, within a group, two electrodes of successive lengths having the same length ratio.
A digital phase modulator for use in a fiber optic gyroscope is taught in WO-A-98/03895. In such modulator, the same pulse response is obtained in both light directions. According to an embodiment, the modulation electrodes, each rotated by 180 degrees in relation to one another, are arranged along the two waveguide arms that are disposed opposite a common counterelectrode.
Since technical requirements focus on resolution that is further increased by a 16-bit phase modulator, for example, in order to reduce the quantization error and, therefore, the quantization noise in a fiber optic gyroscope, a higher-bit design must be created without adversely changing the overall length.
In addition to applications of digital phase modulators in reciprocal interferometers (fiber optic gyroscope, fiber optic current sensor), applications are also possible in integrated optical amplitude modulators (Mach-Zehnder interferometers). Such digital modulators can be applied in satellite tracking for optical transmission in space or in digital laser exposure devices (digital generation of color grades). A further application is a digital frequency modulator in a laser Doppler anemometer. A phase modulator is, at the same time, a frequency modulator (generation of side bands). In general, it is possible to implement all of those applications that have so far used merely analog phase, frequency or amplitude modulation in connection with an integrated optical modulator.